(2.0 E)
5-8
Load safety factors
Rigging accessories are safety critical components which can, by
their failure, lead to injury or death. To maximise safety and to
reduce the risk of the possible consequences of failure, there exist
various recommendations which attempt to govern the safe design
and construction of rigging components. Unfortunately, such
recommendations vary from country to country and are frequently
adaptations of building or construction safety codes which were
never intended to specifically cover loudspeaker rigging.
A further complicating factor, frequently a result of insurance
requirements, is that different load safety factors may be demanded
for different applications. For example, the load safety factor of a
rigging component suitable for a fixed, static application may be
regarded as insufficient for a mobile application.
What exactly is a load safety factor?
It is accepted general engineering practice to design and build
components to have a minimum 1.25X load safety factor. That
means a component or assembly that can resist without breaking, a
load 25% greater than its specified safe working load (SWL).
Load safety factors are tailored to the estimated level of risk
presented by a particular application - the higher the estimated
level of risk, then the higher the load safety factor demanded.
Because of the high level of risk associated with the possible failure
of rigging, load safety factors much greater than 1.25X are
demanded. The EU Machinery Directive for example, specifies a 4X
load safety factor for shackles. This means that a shackle specified
for a SWL of 500 kg should be able to resist a 2000 kg load without
breaking.
Even more stringent load safety factors than those specified under
EU legislation can apply to applications where rigging is used. One
of the most demanding safety codes, the one used to derive the
high load safety factors for d&b rigging accessories, is the German
BGV C1 safety code.
Section §9 of Unfallverhütungsvorschrift Veranstaltungs- und
Produktionsstätten für szenische Darstellung (BGV C1 GUV 6.15)
draws a distinction between components used to rig a fixed, static
load and components used to rig a moving load. Different load
safety factors are required for otherwise similar components
depending upon whether they are used with a fixed or with a
moving load. A higher load safety factor being required for rigging
subject to the higher stress of dynamic loading.
The recommendations are met if it can be shown that:
−
The SWL of components such as ropes and slings used as
rigging for a fixed load does not exceed 1/10 of their
calculated ultimate stress
and
−
The SWL of components such as ropes and slings used as
rigging for a moving load does not exceed 1/12 of their
calculated ultimate stress ...
